Two-tube shock absorbers are known, as for example from my copending U.S. patent application Ser. No. 323,890 filed Nov. 23 1981, now U.S. Pat. No. 4,428,566 and from my earlier U.S. Pat. No. 3,749,210 as well as from U.S Pat. Nos. 2,943,711, 3,661,236, and 4,005,769, German patent document No. 2,912,902, and German utility model No. 7,619,540. A standard such shock absorber has an outer tube centered on an upright axis and forming with an inner tube an outer gas/liquid chamber. A piston rod inside the inner tube forms therewith an inner liquid chamber that can be pressurized by axial displacement of this piston rod and of the piston attached to it that rides on the inner wall of the inner tube. A foot valve allows limited flow between the inner and outer chambers. The inner chamber is completely filled with liquid, and the outer chamber is partially filled with this liquid and has a pressurized head of air above the liquid.
Normally the piston rod passes out of the absorber through a guide-seal assembly that incorporates at least one hard synthetic-resin guide bushing and inner and outer soft seal rings that axially flank this bushing. The inner seal protects the outer seal against the high pressure reigning in the inner chamber as the piston moves axially up. The guide-seal assembly is formed with a passage communicating between a compartment formed between the inner and outer seals and the outer chamber. A check valve in the guide-seal assembly permits flow from the inner chamber to the outer chamber for bleeding air from the inner chamber.
The inner seal in an arrangement such as described in above-cited German utility model No. 7,619,540 is spaced axially down from the guide bushing, normally being provided in a further inner tube extending down from the guide-seal assembly. This inner seal normally lies below the liquid level in the outer chamber. As some lateral bending of the piston rod is common, this inner seal and the elements carrying it must normally be built to permit limited radial shifting. Such construction not only leads to premature wear for this inner seal, which must be able to resist high pressure, but also makes lubrication of the guide bushing and outer seal difficult.
In my copending U.S. patent application Ser. No. 323,890 filed Nov. 23, 1981, now U.S. Pat. No. 4,428,566, I describe a hydropneumatic shock absorber comprising, as is known in the art, an outer tube centered on an upright axis, an inner tube coaxially spaced inside the outer tube and forming therewith an outer gas/liquid chamber, a piston rod inside the inner tube, generally coaxial with the tubes, and forming with the inner tube an inner liquid chamber, and a piston carried on the rod and radially outwardly engaging the inner tube, whereby axial displacement of the piston and rod relative to the tubes in one direction pressurizes the inner chamber. A foot valve between the inner and outer chambers permits limited liquid flow therebetween. A body of liquid fills the inner chamber and partially fills the outer chamber. An annular plug is fixed at the upper axial ends of the tubes and axially upwardly blocks the inner and outer chambers. A guide bushing in the plug surrounds the piston rod and an annular outer seal in the plug surrounds the piston rod above the bushing and forms with the bushing and plug an annular compartment surrounding the rod. According to this invention, the plug is formed with an inner passage having one end opening into the compartment and another end opening into the inner chamber and with an outer passage having one end opening into the compartment and another end opening into the outer chamber. A porous body is provided in the inner passage and a check valve in the outer passage permits flow only from the compartment into the outer chamber. An annular inner seal in the plug and surrounds the piston rod immediately below the bushing.
With this arrangement the compartment is always filled with oil, so that the outer seal as well as the guide bushing are continuously lubricated, thereby improving the sealing performance of the outer seal as well as the guiding performance of the bushing. Since the inner seal is directly beneath the guide bushing in the plug assembly it need not be set up to compensate for lateral bending or other displacement of the piston rod. Furthermore since the inner seal is not axially shiftable so it can act as a check valve it can seal much more tightly at the high pressures it will be exposed to.
In order to keep the seal of the known systems lubricated it must be provided relatively low in the structure. This necessitates the use of a relatively long and expensive seal and guiding arrangement.